18 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 13 



through March). The dominating factor is the northwest winds, which 

 pour around the islands for the majority of the days of the year. This 

 increases aridity over what might be expected on the basis of rainfall 

 and temperature (60°F. mean annual) only. However, transpiration 

 and general aridity are tempered by high humidity and frequent fogs. 

 The climate borders on a fog or maritime desert over the southern por- 

 tion of San Clemente Island, the southern-most of the group. 



There is considerable variation in climate locally, both on individual 

 islands and between different islands. This is explainable primarily in 

 terms of the predominating direction of air flow. Physical conditions 

 vary greatly whether a given area is on a windward or leeward side. 

 This is expressed by the vegetation. Windward slopes commonly are 

 covered with grass and forbs ( PI. 4, fig. 11), or stunted wind-tolerant 

 shrubs, while protected slopes support shrub and tree communities. Some 

 of the smaller islands that have been greatly disturbed by man have been 

 nearly denuded of cover and top soil by the erosive action of wind. 

 This is true of San Miguel, which catches the full force of the north- 

 westers, and its sterile mobile sands are being discharged into the lee sea. 



The vegetation of the islands has been greatly altered by man. Both 

 the aborigine, who burned and cut the native plants, and the modern 

 European segregates, who burned, cut, cleared, and pastured, have left 

 only remnants of the virgin cover. Many of the native plants are intoler- 

 ant to grazing, and as they were destroyed, the aggressive weeds were 

 introduced and have persistently spread. While considerable has been 

 published about the flora of the islands, very little is on record regarding 

 the vegetation. The only study of the plant ecology, that I have been 

 able to find, which gives an adequate account of the vegetation is a doc- 

 toral thesis by M. B. Dunkle (1944). Between 1939 and 1943 he made 

 repeated visits to the islands and his introductory statement (1944:128- 

 129) is a concise general picture of the island vegetation. 



^'Varied as are the different islands in topography and climate, they 

 possess certain basic similarities. The western slope of all the islands, 

 except San Nicolas which consists of barren sand dunes in this area, are 

 covered with grasses, low forbs, suffrutescent perennials, and a few low 

 wind-tolerant shrubs. The eastern slopes, except on the smaller islands, 

 are quite generally covered with chaparral, or shrub and tree savannas. 

 The canyons, which afford protection from the wind, usually have more 

 or less shrubby growth on their slopes. This varies from an Opuntia lit- 

 toralis association, through various facies of the coastal sage brush associa- 

 tion, to chaparral on the larger islands. On these larger islands, and on 



